This invention relates, in general, to semiconductor processing and more particularly, to a method of forming improved silicon-on-sapphire devices.
The present day method of forming islands of silicon on an insulating substrate, such as sapphire, utilizes an anisotropic etch utilizing a potassium hydroxide (KOH) solution. Since this is an anisotropic etch, by its nature, islands will be produced having sloped sides wherein the base of the island is both longer and wider than the top of the island. However, when an island formed in this manner is utilized to fabricate a semiconductor device, without any further treatment, it has been noted that the gate dielectric breakdown voltage is consistently lower than that of an edgeless device. This phenomenon has been investigated in detail and appears to be caused by the presence of a peak or point at the upper edge of the island which peak is produced as a result of the anisotropic etch. For a more detailed discussion of the structure near the top edge of the island, attention is directed to an article entitled "The Study of Microcircuits by Transmission Electron Microscopy" by W. E. Ham, et al., RCA Review, Vol. 38, September 1977, pp. 351-389. Particular attention is directed to FIGS. 12-15 of the above-referenced article wherein a high magnification cross-section transmission electron microscopy photograph shows an image of the island edge after an anisotropic etch. It is theorized that the presence of the sharp edge (interface tilt) adjacent the upper surface of the silicon island also causes any deposited oxide formed around this point to be thinner than at other parts of the island. Consequently, the low gate breakdown voltage would most likely occur at the top edge, at the undesirable point.
One prior art method used to remove the objectionable point, is to heavily oxidize only the side of the island where the point is formed. However, since thermal oxidation of a given volume of silicon produces approximately twice that value of oxide, it will be seen that to produce an oxide thickness of approximately 1000 angstroms, only 500 angstroms of silicon would be oxidized. In many instances this would not be enough to remove the objectionable point and, in some instances, would introduce still another undesirable feature. A heavy oxidation, only on the sides of the island, would provide a noticeable separation or lifting of silicon oxide from the upper surface and thus produce a bird beak effect.
Still another attempt to solve the problem brought about by the presence of the interface tilt or point is detailed in U.S. Pat. No. 4,277,844, issued to S. T. Hsu on July 14, 1981 and assigned to the same assignee as the subject application. This latter reference acknowledges the presence of the interface tilt and provides the island with successive masking layers then unmasks only the area immediately adjacent to the objectionable projection tilt in order to selectively, heavy oxidize only that portion of the island containing the interface tilt. This process, which is successful in removing the interface tilt, also produces a bird beak configuration.